This invention relates generally to the field of air conditioning and deals more particularly with a low temperature air conditioning system and method for use in relatively large commercial buildings.
The heating and cooling of large buildings such as multiple story office buildings is normally accomplished by circulating conditioned air through ventilating ducts that extend throughout the building. The air used to cool the building is usually supplied at about 55.degree. F. In a variable air volume system, either the ducts or the air diffusers which discharge the conditioned air into the rooms of the building are equipped with flow control devices which permit the flow of conditioned air into each room to be individually controlled. In this manner, each room can be independently controlled as to its temperature and, at the same time, operating efficiencies and low costs are achieved due to the use of a single air handling unit to supply a number of different areas or floors of the building.
Despite the recognized advantages of variable volume systems, they are not wholly without problems. Because the conditioned air is normally supplied at about 55.degree. F., it is necessary to provide relatively large ducts in order to adequately deliver the conditioned air throughout the building. The need for a large central air shaft and large ventilating ducts reduces the amount of useful space available in the building for rent or other purposes. It is also necessary for the central fan to pressurize the entire duct system and also handle all of the apparatus loss (due to the presence of filters, coils, sound absorbers, mixing dampers and other obstructions in the ductwork). The fan of a conventional variable air volume system in a typical office building must develop a pressure of about 5-6 inches W.G. Therefore, the fan motor must have considerable horsepower capacity which significantly adds to both the initial cost and to the ongoing operating costs. The need for a relatively large cooling coil further contributes to the initial cost of the air conditioning equipment.
Another type of air distribution system known as a high pressure induction system has long been used in office towers. The induction systems circulate colder air and moderate the air temperature in the ducts by adding return air taken from the conditioned space. However, it is impractical in the induction system to have a pressure of more than about 0.3 inch W.G. in the secondary air path or downstream from the point where the cold primary air and the secondary air are mixed, because the induction process degrades rapidly with increasing pressure. This low secondary air or downstream pressure is a practical limitation which makes it impossible to use variable volume terminal units in an induction system because the downstream pressure is too low to drive air through the terminal.
The use of fans to mix secondary return air with varying volume primary air is also known. However, the fan powered boxes that are capable of use in variable volume systems must fit above the ceiling in a commercial building, and fans of this type have not been able to develop enough pressure to adequately drive the conditioned air through a variable volume terminal unit. Consequently, it has not been practical to use variable volume terminals downstream of fan powered boxes.